The present invention relates to a multilayer catalyst for preparing phthalic anhydride which has a plurality of catalyst layers arranged in succession in the reaction tube, with the individual catalyst layers having alkali metal contents which decrease in the flow direction. The present invention further relates to a process for the oxidation of naphthalene or o-xylene/naphthalene mixtures over such a multilayer catalyst and the use of such multilayer catalysts for the oxidation of naphthalene or o-xylene/naphthalene mixtures to phthalic anhydride.
Many carboxylic acids and/or carboxylic anhydrides are prepared industrially by catalytic gas-phase oxidation of hydrocarbons such as benzene, the xylenes, naphthalene, toluene or durene in fixed-bed reactors. In this way, it is possible to obtain, for example, benzoic acid, maleic anhydride, phthalic anhydride, isophthalic acid, terephthalic acid or pyromellitic anhydride. In general, a mixture of an oxygen-comprising gas and the starting material to be oxidized is passed through tubes in which a bed of a catalyst is present. To regulate the temperature, the tubes are surrounded by a heat transfer medium, for example a salt melt.
Catalysts which have been found to be suitable for these oxidation reactions are coated catalysts in which the catalytically active composition is applied in the form of a shell to an inert support material such as steatite. In general, titanium dioxide and vanadium pentoxide are used as catalytically active constituents of the catalytically active composition of these coated catalysts. Furthermore, small amounts of many other oxidic compounds which act as promoters to influence the activity and selectivity of the catalyst can be comprised in the catalytically active composition.
It has been found to be particularly advantageous to use different catalysts in the catalyst bed which differ in terms of their catalytic activity and/or the chemical properties of their active composition. When using two reaction zones, the catalyst preferably used in the first reaction zone, i.e. the reaction zone located nearest the inlet for the reaction gas, has a somewhat lower catalytic activity than the catalyst present in the second reaction zone, i.e. the reaction zone nearest the gas outlet. In general, the reaction is controlled by means of the temperatures set so that the major part of the aromatic hydrocarbon comprised in the reaction gas is reacted with maximum yield in the first zone. Preference is given to using three- to five-layer catalyst systems, in particular three- and four-layer catalyst systems.
The oxidation of o-xylene to phthalic anhydride (PAn) over vanadium oxide/titanium dioxide catalyst systems is usually carried out at air flows of about 4 standard m3/h and o-xylene loadings of up to 100 g/standard m3. For the oxidation of o-xylene/naphthalene mixtures, the catalysts have typically been developed so that they are particularly well suited to a particular o-xylene/naphthalene mixing ratio or a narrow range of o-xylene/naphthalene mixing ratios. If the o-xylene/naphthalene ratio is altered significantly, either the PAn yield decreases drastically, the product quality becomes significantly poorer and/or the operative life of the catalyst is adversely affected. This is particularly pronounced at high loadings of o-xylene or naphthalene. The higher the total loading of o-xylene and naphthalene, the smaller the range of possible o-xylene/naphthalene ratios.
EP 539878 describes a process for the oxidation of o-xylene/naphthalene mixtures over a two-layer catalyst. Weight ratios of from 10/90 to 90/10% are used, and the maximum total loading in a single pass is 70 g/standard m3 at a space velocity (GHSV) of 3000 h−1. The PAn yields are in the range from 98.5 to 111.5% by weight, depending on the catalyst and o-xylene/naphthalene mixing ratio.
In EP 744214, PAn yields of only 101% by weight were achieved at a naphthalene loading of 80 g/standard m3 and 4 standard m3/h of air.
In the case of a two-layer catalyst as described in EP 1082317, a PAn yield of 110% by weight was achieved at from 65 to 80 g/standard m3 and a 75% by weight/25% by weight o-xylene/naphthalene mixture and 4 standard m3/h of air. Variation of the o-xylene/naphthalene ratio was not carried out.
The two-layer catalysts in EP 286448 were operated using 70 g/standard m3 of naphthalene and a GHSV of 3000 h−1. However, the o-xylene/naphthalene ratios were varied only from 100:0 to 50:50 or from 50:50 to 0:100 for individual catalysts. Wider variation of the mixing ratios using the same catalyst is not described.
Catalysts having more than two catalyst layers have been described for the oxidation of o-xylene to phthalic anhydride even at very high loadings of o-xylene of up to 100 g/standard m3 at 4 standard m3/h of air. An example is a three-layer catalyst system for the oxidation of o-xylene to PAn as described in EP 1084115. However, these catalysts are not suitable for the oxidation of o-xylene/naphthalene mixtures at total loadings of at least 80 g/standard m3 at about 4 standard m3/h of air with a wide variation of the o-xylene/naphthalene ratio.
There is a continuing need for catalysts for gas-phase oxidations which give a very high conversion at high selectivity.